To examine the plasma levels of MPO in oligoarthritis and polyarthritis subtypes of JIA in comparison with healthy age-matched controls. Thirty-eight JIA patients (25 girls and 13 boys) aged 9.1–11.8 years and 23 healthy controls (8 girls and 15 boys) participated in the study. Twenty-one patients had oligoarthritis (8 with extended oligoarthritis) and 17 had polyarthritis (among them three were seropositive). The plasma concentration of MPO was measured by the ELISA technique (OxisResearchTM, BIOXYTECH® MPO-EIATM, Portland, OR USA). The mean plasma concentration of MPO in the JIA group was significantly higher than in the control group (76.6±24.8 µg/L versus 62.7±15.6 µg/L; p=0.01). Patients with polyarthritis presented a significantly higher mean plasma MPO level than patients with oligoarthritis (81.3±25.6 µg/L and 62.1±27.1 µg/L, respectively; p=0.02). Different subtypes of JIA may have different MPO-related backgrounds. MPO is a new potent inflammatory marker. Patients with polyarthritis have higher mean plasma MPO levels than patients with oligoarthritis and may therefore have an enhanced risk for subclinical oxidative stress-related atherogenic promotion.